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authorRoland McGrath <roland@redhat.com>2008-04-03 14:18:55 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-04-03 15:42:43 -0700
commit4ba51fd75cc3789be83f0d6f878dabbb0cb19bca (patch)
tree7e1ab105022a571facd81b7fd8040b96e58d73af
parent2eccd6f65a0d4844318b1e30755cafd063833908 (diff)
x86 ptrace: avoid unnecessary wrmsr
This avoids using wrmsr on MSR_IA32_DEBUGCTLMSR when it's not needed. No wrmsr ever needs to be done if noone has ever used block stepping. Without this change, using ptrace on 2.6.25 on an x86 KVM guest will tickle KVM's missing support for the MSR and crash the guest kernel. Though host KVM is the buggy one, this makes for a regression in the guest behavior from 2.6.24->2.6.25 that we can easily avoid. I also corrected some bad whitespace. Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--arch/x86/kernel/step.c11
1 files changed, 7 insertions, 4 deletions
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 9d406cdc847..071ff479823 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -140,6 +140,9 @@ static int enable_single_step(struct task_struct *child)
*/
static void write_debugctlmsr(struct task_struct *child, unsigned long val)
{
+ if (child->thread.debugctlmsr == val)
+ return;
+
child->thread.debugctlmsr = val;
if (child != current)
@@ -165,11 +168,11 @@ static void enable_step(struct task_struct *child, bool block)
write_debugctlmsr(child,
child->thread.debugctlmsr | DEBUGCTLMSR_BTF);
} else {
- write_debugctlmsr(child,
- child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
+ write_debugctlmsr(child,
+ child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
- if (!child->thread.debugctlmsr)
- clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
+ if (!child->thread.debugctlmsr)
+ clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
}
}